In the case of membrane separation process using a membrane such as an ultrafiltration membrane or a reverse osmosis membrane, solid materials in a raw liquid adhere. to the inner surface of the membrane with the lapse of time and a so-called membrane contamination proceeds. As a result, deterioration in permeation performance of the membrane is unavoidable. Therefore, the membrane must be periodically washed to recover the permeation performance of the membrane.
The efficiency of washing can be evaluated by the degree of recovery of the permeation performance of the membrane, viz., the ratio of the amount of the membrane permeated liquid after washing the membrane to the initial amount of the membrane permeated liquid. Any criterion has not been conventionally established to determine the efficiency of washing and this efficiency factor is appropriately determined in accordance with various conditions such as the manner of washing, the degree of contamination of the membrane, the period of washing, etc.
The present inventors have found that there is the relationship such that the higher the efficiency of washing is, the lcwer the rate of membrane contamination becomes, between the efficiency of washing (i.e., the degree of the amount of contaminants adhered to the surface of the membrane after washing) and the rate of membrane contamination after washing (i.e., reduction in liquid permeation rate with the lapse of time). For example, referring to FIG. 2, curves A, B and C each shows the state of reductron in liquid permeation rate after washing when the liquid permeation rate decreases to 9.462 lpm (liter per minute; hereinafter the same) in an ultrafiltration apparatus using a tubular permeable membrane having the initial amount of the membrane permeated liquid of 30.28 lpm. That is, the curves A, B, and C show the states after washing the membrane so as to obtain the liquid permeation rate of 21.763 lpm (the degree of recovery: about 72%), after washing the membrane so as to obtain the liquid permeation rate of 24.60 Zpm (the degree of recovery: about 81%), and after washing the membrane so as to obtain the liquid permeation rate of 28.387 lpm (the degree of recovery: about 94%), respectively. It is apparent from the results that slight increase in the efficiency of washing greatly contributes to preventing the liquid permeation rate from lowering. For example, on comparison between the curves B and C shown in FIG. 2, in the case of the curve B, the degree of recovery is 81% and the degree of reduction in the liquid permeation rate after washing (30 days after) is 42% (the ratio of the lowered liquid permeation rate after 30 days to the liquid permeation rate immediately after washing). On the other hand, in the case of the curve C, the degree of recovery is 94% and the degree of reduction in the liquid permeation rate after washing is 13%. Therefore, the degree of reduction in the liquid permeation rate can be decreased about 30% by only increasing the degree of recovery by 13%.
As described above, in the membrane washing, the more the degree of recovery in the permeation performance of the membrane is increased, the more the degree of membrane contamination after washing can be decreased, so that it is possible to make the intervals of washing of the membrane longer to thereby minimize the frequency of washing. This is convenient in the maintenance of membrane.
A method in which a chemical agent is filled in a tubular membrane to dissolve contaminants adhered to the membrane is conventionally known as a method for washing a tubular permeable membrane. In this method, however, it is extremely difficult to reach the above-described degree of recovery near 100%. Therefore, a further methcd is known in which after washing the membrane with the chemical agent, a washing ball such as sponge ball or the like is introduced into the tubular membrane to run it within the tubular permeable membrane by a fluid back pressure. In this method, contaminants adhered to the surface of the membrane are removed by shearing force (hereinafter referred to as "rub-washing force") caused between the washing ball and the membrane surface.
The rub-washing force & is expressed as follows: EQU .tau.=A.sub.1 (P.sub.1 -P.sub.2)/2A.sub.2
wherein P.sub.1 and P.sub.2 represent upstream and downstream fluid pressures acting onto the washing ball, respectively, and A.sub.1 and A.sub.2 represent contacting areas between the ball and the fluid and between the ball and the membrane, respectively. In order to increase the rub-washing force, the fluid pressure must be increased. However, a limit exists to increase the pressure P.sub.1 in view of the pressure resistance of the membrane. Further, there is an inconvenience that if the pressure P.sub.1 is increased, the flow rate correspondingly increases to thereby cause a difficulty in liquid operation. Thus, a limit exists to increase the rub-washing force. In this case, it is also difficult to reach the above-described degree of recovery near 100%.
Thus, it is difficult in the conventional method for washing a surface of a tubular permeable membrane to reach the degree of recovery of the permeation performance of the membrane near 100% and the practical upper limit is at most 80%.
If the degree of recovery can be increased even several %, the lowering of the liquid permeation rate with the lapse of time after washing or the progress of membrane contamination can be effectively prevented, as described above. In the prior art methods, however, such a technical advantage has not been recognized.
The present inventors have recognized such a technical advantage and made various investigations on a washing method which is capable of reaching the degree of recovery of the permeation performance of a membrane at least 90%, preferably near 100%.
A method in which in the above-described washing ball system, a pushing rod to directly transmit external force to a washing ball is used instead of fluid pressure as an external force which is a washing force source is known as such a method, and this method is known as a washing means in the field of elongated or deep vessels.
If this washing means can be utilized to wash a tubular permeable membrane, it will be possible to perform washing in which the permeation performance of the membrane can be substantially recovered completely. As a result, the progress of membrane contamination after washing can be remarkably improved, and the total effect obtained by the combinaticn with the tubular permeable membranes can be unexpectedly raised.